Modern web pages can be generated by executing multiple program modules and functions in response to an almost limitless variety of user input and/or interactions. The appearance of a web page can change depending on the characteristics of the user device accessing the web page. For example, different user devices may have different form factors, resolution, and browser applications. A programmer or developer changing one line of code, function, or module may not be able to determine the impact on other portions of the code.
Almost a limitless number of variations can be created for an individual web page making quality control very difficult. For example, a table on a web page could be populated with data drawn from one or more knowledge bases in response to a query. Images and advertisements could be presented from other sources. A search engine might generate search results in multiple formats. In each case, a single error in the code can cause display abnormalities on the web page. For example, a margin could change, a color of text could change, a function could fail to execute, and such. Catching and fixing these errors before placing updated code into production remains a challenge.